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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_asio.reference.CancellationHandler"></a><a class="link" href="CancellationHandler.html" title="Cancellation handler requirements">Cancellation
      handler requirements</a>
</h3></div></div></div>
<p>
        A value <code class="computeroutput"><span class="identifier">h</span></code> of a cancellation
        handler class should work correctly in the expression <code class="computeroutput"><span class="identifier">h</span><span class="special">(</span><span class="identifier">t</span><span class="special">)</span></code>,
        where <code class="computeroutput"><span class="identifier">t</span></code> is a value of type
        <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">cancellation_type</span></code>.
      </p>
<h5>
<a name="boost_asio.reference.CancellationHandler.h0"></a>
        <span class="phrase"><a name="boost_asio.reference.CancellationHandler.examples"></a></span><a class="link" href="CancellationHandler.html#boost_asio.reference.CancellationHandler.examples">Examples</a>
      </h5>
<p>
        A free function as a cancellation handler:
      </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">cancellation_handler</span><span class="special">(</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">cancellation_type</span> <span class="identifier">type</span><span class="special">)</span>
<span class="special">{</span>
  <span class="special">...</span>
<span class="special">}</span>

<span class="identifier">slot</span><span class="special">.</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">cancellation_handler</span><span class="special">);</span>
</pre>
<p>
        A cancellation handler function object:
      </p>
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">cancellation_handler</span>
<span class="special">{</span>
  <span class="special">...</span>
  <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">()(</span>
      <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">cancellation_type</span> <span class="identifier">type</span><span class="special">)</span>
  <span class="special">{</span>
    <span class="special">...</span>
  <span class="special">}</span>
  <span class="special">...</span>
<span class="special">};</span>

<span class="identifier">cancellation_handler</span><span class="special">&amp;</span> <span class="identifier">h</span> <span class="special">=</span> <span class="identifier">slot</span><span class="special">.</span><span class="identifier">assign</span><span class="special">(</span><span class="identifier">cancellation_handler</span><span class="special">{</span> <span class="special">...</span> <span class="special">});</span>
</pre>
<p>
        A lambda as a cancellation handler:
      </p>
<pre class="programlisting"><span class="keyword">auto</span><span class="special">&amp;</span> <span class="identifier">h</span> <span class="special">=</span> <span class="identifier">slot</span><span class="special">.</span><span class="identifier">assign</span><span class="special">(</span>
    <span class="special">[](</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">cancellation_type</span> <span class="identifier">type</span><span class="special">)</span>
    <span class="special">{</span>
      <span class="special">...</span>
    <span class="special">});</span>
</pre>
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